Mendelian link questions

Molecular Phenotype:

"phenotype" is viewed as the presence of a protein or DNA band in a gel. Many molecular phenotypes are "codominant" even if the directly visible trait is not: both "traits" show. Partial dominance, one allele instead of 2 is producing enzyme, so on a gel, a protein band is half as intense

Disjunction:

(separation of chromosomes) of homologues is a random event, that is any combination of maternal and paternal chromosomes can separate together.

A human female "carrier" who is heterozygous for the recessive, sex-linked trait red color blindness, marries a normal male. What proportion of their female progeny will show the trait?

0 Half the sons would be expected to inherit the color blindness allele from their mother. Half the daughters would be carriers like their mothers.

In a cross between a pure bred, red-eyed female fruit fly and a white-eyed male, what percent of the male offspring will have white eyes? (white eyes are X-linked, recessive)

0%

In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will have white eyes? (White eyes are X-linked, recessive)

0%

In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits?

1/16 1/4 of the gametes of each parent will be ab. The fraction of the offspring homozygous for both recessive traits will be 1/4 times 1/4, or 1/16.

In Mendel's experiments, if the gene for tall (T) plants was incompletely dominant over the gene for short (t) plants, what would be the result of crossing two Tt plants?

1/4 would be tall; 1/2 intermediate height; 1/4 short The heterozygous offspring (Tt) would be of intermediate height.

The expected phenotypic ratio of the progeny of a SsYy x ssyy test cross is

1:1:1:1. SsYy, ssYy, Ssyy, ssyy are predicted to occur in a ratio of 1:1:1:1.

A genetic cross between two F1-hybrid pea plants having yellow seeds will yield what percent green-seeded plants in the F2 generation? Yellow seeds are dominant to green.

25% Among the F2 plants of a Yy x Yy cross, 25% will be yy with the recessive, green-seeded phenotype.

A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What will be the distribution of these two alleles in this plant's gametes?

25% of gametes are SY; 25% of gametes are Sy; 25% of gametes are sY; 25% of gametes are sy. Alleles of different genes are assorted independently of each other during the formation of gametes.

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

3/4 One fourth of the offspring will be homozygous dominant (SS), one half will be heterozygous (Ss), and one fourth will be homozygous recessive (ss).

Following a SsYy x SsYy cross, what fraction of the offspring are predicted to have a genotype that is heterozygous for both characteristics?

4/16 Four out of 16 possible combinations of alleles yield the SsYy genotype. They are: SY and sy, sY and Sy, Sy and sY, SY and sy.

A genetic cross between two F1-hybrid pea plants for spherical seeds will yield what percent spherical-seeded plants in the F2 generation? (Recall, spherical-shaped seeds are dominant over dented seeds.)

75% Only 25% of F2 plants will have the recessive phenotype.

Two unlinked loci effect mouse hair color. AA or Aa mice are agouti. Mice with genotype aa are albino because all pigment production is blocked, regardless of the phenotype at the second locus. At the second locus, the B allele (agouti coat) is dominant to the b allele (black coat). What would be the result of a cross between two agouti mice of genotype AaBb?

9 agouti: 3 black: 4 albino This is a variant of the normal 9:3:3:1 ratio for a normal dihybrid cross without epistasis.

Sex Linked:

A gene coded on a sex chromosome, such as the X-chromosome linked genes of flies and man.

What are the possible blood types of the offspring of a cross between individuals that are type AB and type O? (Hint: blood type O is recessive)

A or B A cross between individuals that are of genotype IAIBx ii can yield offspring that are either IAi or IBi. Their blood type will be A or B

Explain how the Bombay phenotype is the result of epistasis.

A second gene, I^h codes for the base sugar chain to which the A and B sugars are added. A rare mutation I^h prevents proper formation so that the A and B sugars cannot be added even though the enzyme for doing that is being made. A second gene is masking the normal phenotype.

A white-eyed female fruit fly is crossed with a red-eyed male. Red eyes are dominant, and X-linked. What are the expected phenotypes of the offspring?

All of the females will have red eyes; all of the males will have white eyes. All of the females are red-eyed and heterozygous. All of the males are white-eyed and hemizygous.

In Mendel's "Experiment 1," true-breeding pea plants with spherical seeds were crossed with true-breeding plants with dented seeds. (Spherical seeds are the dominant characteristic.) Mendel collected the seeds from this cross, grew F1-generation plants, let them self-pollinate to form a second generation, and analyzed the seeds of the resulting F2 generation. The results that he obtained, and that you would predict for this experiment are:

All the F1 and 3/4 of the F2 generation seeds were spherical. All of the F1 plants were true hybrids with a phenotype of Ss. The recessive trait reappears in the F2 generation.

In Mendel's experiments, the spherical seed character (SS) is completely dominant over the dented seed character (ss). If the characters for height were incompletely dominant, such that TT are tall, Tt are intermediate and tt are short, what would be the phenotypes resulting from crossing a spherical-seeded, short (SStt) plant to a dented-seeded, tall (ssTT) plant?

All the progeny would be spherical-seeded and intermediate height. All of the offspring would have the genotype of SsTt. Mendel selected traits to study that did not display partial dominance. Could you predict the result of an SsTt x SsTt cross if one or both traits dispayed partial dominance? There would be 9 different phenotypes if both traits showed partial dominance.

Heterozygous

Alleles are different (1 of each)

Allele:

An alternative form of a gene; for a given gene, many alleles may exist in the gene pool of the species. The different forms of a gene Y and y are different alleles of the same gene that determines seed color. Alleles occupy the same locus, or positions on chromosomes.

Two unlinked loci effect mouse hair color. CC or Cc mice are agouti. Mice with genotype cc are albino because all pigment production and deposition of pigment in hair is blocked. At the second locus, the B allele (black agouti coat) is dominant to the b allele (brown agouti coat). A mouse with a black agouti coat is mated with an albino mouse of genotype bbcc. Half of the offspring are albino, one quarter are black agouti, and one quarter are brown agouti. What is the genotype of the black agouti parent?

B. BbCc This is the only possibility for this combination of offspring.

Homozygous

Both alleles are the same

Mono-hybrid Cross:

Cross involving parents differing in only one trait.

In a dihybrid cross, SsYy x SsYy, what fraction of the offspring will be homozygous for both traits?

D. 1/4 There are four different genotypes that are homozygous for both traits. These are SSYY, SSyy, ssYY, and ssyy.

Homogametic:

Denoting the sex that has sex chromosomes that do not differ in morphology, resulting in one kind of gamete

Dominant:

Dominance is a situation in a diploid organism in which one allele masks another. Dominant does not mean most common. It does not mean that it will outcompete another allele in the population over time. It can be co-dominance (neither trait masks the other) or partial dominance ( both traits are partially seen, neither completely masks the other).

Describe complementary experiments and observations that demonstrated that the SRY region is responsible for maleness.

Evidence that SRY determines maleness-sex determination region Y. Crossing over in meiosis- Observations males with two X chromosomes; SRY found on one, Females with XY; SRY is missing from Y. In transgenic mice remove SRY from chromosome, mice are XY but female, Reciprocal experiment also done XX mouse with SRY region is male

When true-breeding tall stem pea plants are crossed with true-breeding short stem pea plants, all of the _________ plants, and 3/4 of the __________ plants had tall stems. Therefore, tall stems are dominant.

F1, F2. The F1 plants are all Tt hybrids. The recessive trait (tt) reappears in the F2 generation in about 25% of the plants.

Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a women's grandparents could not be the source of any of the genes on either of her X-chromosomes?

Father's Father. The father's father contributes only the Y chromosome to his sons, and subsequently to his grandsons.

A female Drosophila of unknown genotype was crossed with a white-eyed male fly, of genotype (w = white eye allele is recessive, w+= red-eye allele is dominant.) Half of the male and half of the female offspring were red-eyed, and half of the male and half of the female offspring were white-eyed. What was the genotype of the female fly?

Female must be heterozygous because she can transmit either the red eye or white eye allele to her sons and daughters. Therefore the female must be a heterozygous, Xw+ Xw.

Linkage:

Genes that are inherited together on the same chromosome. Three inheritance patterns are possible: non-linkage, partial linkage, and complete linkage.

A human female "carrier" who is heterozygous for the recessive, sex-linked trait causing red-green color blindness (or alternatively, hemophilia), marries a normal male. What proportion of their male progeny will have red-green color blindness (or alternatively, will be hemophiliac)?

Half the sons would be expected to inherit the allele from their mother and be afflicted because they are hemizygous. Half the daughters would be carriers like their mothers.

True - Breeding:

Homozygous for the true-breeding trait.

Reciprocal Cross:

Matings with traits swapped in parents. Matings with traits swapped in parents. Using male and female gametes for two different traits, alternating the source of gametes.

Mendel's Law of Segregation:

Mendel's First Law. During formation of gametes, the paired unit factors separate (segregate) so that each gamete gets one of them (at random) Each gamete equally likely to receive either unit.

Mendel's Law of Independence:

Mendel's second law of independent assortment. During gamete formation segregating pairs of alleles assort independently of each other. Paternal traits not always inherited together, maternal traits not always inherited together. Inheritance of one trait is not affected by inheritance of another trait.

How did Thomas Hunt Morgan show that eye color in fruit flies was X linked by doing a reciprocal cross?

Morgans work led to the understanding that genes are located on the chromosomes because inheritance of certain traits corresponded to inheritance of a visibly different chromosome. Inheritance of X linked traits resulted in typical crisscross inheritance: mother to son. Carrier mother passes allele to son who expresses it, passes allele to daughter who carries it.

Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a man's grandparents could not be the source of any of the genes on his Y-chromosome?

Mother's Mother, Mother's Father, and Father's Mother. The Y chromosome is inherited solely from father to son in each generation.

Lethal Alleles:

Mutated genes that are capable of causing death.

Trihybrid Cross

Offspring formed by crossing of parents that differ by three traits

Partial Dominance

One allele partially masks the other.

Epistasis:

One gene masks the expression of a different gene for a different trait. Example: The Bombay phenotype. Example: In the metabolic disease alkaptonuria, a missing enzyme (because of the recessive genotype for that gene) blocks the pathway leading to the breakdown and disposal of excess tyrosine (an amino acid). The result of this condition is (among other things black urine.

Sum Law:

Outcomes of events are independent, but can be accomplished in more than way. Flip a penny and a nickel: odds of 1 head and 1 tail? There are 4 possible outcomes from this flip. 1 head and 1 tail from the penny being heads (1/4), but also from the nickel (1/4): 1/4 +1/4=1/2

Dihybrid Cross:

Parents differ in two traits.

Product Law:

Product law is used to calculate odds of an outcome from independent events. Example Flip a coin heads or tails, 50:50 chance of getting a head. The chance of getting 4 heads in a row 1/2*1/2*1/2=1/16 Example: Odds of round, yellow seeds in a cross of Ww GG*Ww gg: 3/4*4/4=3/4

The gametes of a plant of genotype SsYy should have the genotypes:

SY, Sy, sY, and sy Each gamete will receive either on S or s allele, and either a Y or y allele. All of S or s, and Y or y are possible.

Biological Sex versus Gender

Sexuality (male/female) is physical; gender is psychological/social. Sexual orientation independent of sexuality/gender.

What is the genotype of a red-eyed, yellow-bodied female fruit fly who is homozygous for the eye color allele? Red eyes (w+) and tan bodies (y+) are the dominant alleles. (Both traits are X chromosome linked).

Since the female fly has a yellow body, and the tan body color is the dominant allele, she will be homozygous recessive for yellow body color. Therefore, the female fly will be homozygous for both traits. The genotype is Xw+y Xw+y.

During non-disjunction occurring in anaphase 2 of meiosis, what fails to separate?

Sister chromatids

Why would Mendel's studies have been hard to interpret if he used snapdragons (which show partial dominance) instead of peas, considering the ideas about inheritance prevalent at the time? Why would Mendel have probably figured it out anyway if he did enough crosses?

Snap dragons show partial dominance. They can be homozygous for either red or white. There are more combinations of alleles. He would eventually figured out what crosses to make to get the intended results.

Which of the following genotypes would you not expect to find among the offspring of a SsYy x ssyy test cross:

SsYY Offspring could not be homozygous for the dominant yellow seed color (YY), because one recessive y allele must be inhereited from the ssyy parent.

Which of the following genetic crosses would be predicted to give a phenotypic ratio of 9:3:3:1?

SsYy x SsYy The dihybrid cross was invented by Mendel to discover the independent assortment of alleles during gamete formation.

Expressivity:

The degree to which an individual shows the phenotype. They may not show the whole phenotype.

Penetrance:

The extent to which a dominant trait actually appears in a population- meaning that just because something is dominant, does not mean that it will be expressed the most. It can be high reduced, or low. Just because you inherit a dominant genotype does not mean you will express it. Trait can be highly penetrant, or show reduce penetrance, or low penetrance,( not all with that allele show the condition) - Neurofibromatosis is 100% and polydactyly is about 50% penetrant.

Recessive:

The opposite of dominant. A trait that is preferentially masked.

P Generation:

The parents

F2 Generation:

The second generation of offspring

Heterogametic

The sex that is specified by having two different sex chromosomes

Mendel used "true-breeding" plants in his experiments. Why couldn't he have just used a purple-flowered pea plant without knowing its "family tree"?

There is a chance that the purple-flowered pea plant was heterozygous. He would have needed to know the exact history of the plant to do a proper test cross.

CrissCross Inheritance:

This is due to X-linked traits meaning that the trait is passed from the mother to the son; such as in color blindness and hemophilia. The daughters can be the carriers, while the males are the ones who are affected.

Continuous Variation

Traits usually quantifiable (weighing, etc) Two more genes contribute to phenotype in an additive way. Individual allele either adds to phenotype or doesn't.

Multiple Alleles:

Two or more alleles for a trait.

Human blood type is determined by codominant alleles. There are three different alleles, known as IA, IB, and i. The IA and IB alleles are codominant, and the i allele is recessive. The possible human phenotypes for blood group are type A, type B, type AB, and type O. Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively). A woman with type A blood and a man with type B blood could potentially have offspring with which of the following blood types?

Type A, Type AB, Type B, Type O But if the man was type O rather than type B, offspring of type B and type AB would not be possible.

Sex Chromosome:

Type of chromosome that determines the sex of an individual and directs the development of sexual characteristics. In mammals, the X and Y chromosome.

Sex Limited:

When the gene is not expressed based on the organisms sex. Example: A gene that deals with milk production in a male is not expressed due to the lack of organs

Hemizygous:

With regard to X linked traits, males have only one allele, not two. They are said to be hemizygous.

Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophilac male?

all sons are normal and all daughters are carriers. Daughters inherit a normal allele from their mother and the hemophilia allele from their father. Sons inherit the normal allele from their mother.

Non-disjunction

an error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.

Codominance:

both alleles are expressed: not blended; both equally exist

Quantitative Traits:

different from multiple genes. Seems like it should be the same, but no, also called polygenic traits. Quantitative traits are different in amount not type. Polygenic traits usually show continuous variation.

A test cross is used to determine if the genotype of a plant with the dominant phenotype is homozygous or heterozygous. If the unknown is homozygous, all of the offspring of the test cross have the __________ phenotype. If the unknown is heterozygous, half of the offspring will have the __________ phenotype.

dominant, recessive The test cross was invented by Mendel to determine the genotype of plants displaying the dominant phenotype.

F1 Generation:

first generation of offspring

A genetic cross of inbred snapdragons with red flowers with inbred snapdragons with white flowers resulted in F1-hybrid offspring that all had pink flowers. When the F1 plants were self-pollinated, the resulting F2-generation plants had a phenotypic ratio of 1 red: 2 pink: 1 white. The most likely explanation is:

heterozygous plants have a different phenotype than either inbred parent because of incomplete dominance of the dominant allele. The features of crosses involving incomplete dominance are intermediate phenotype of heterozygous individuals, and parental phenotypes reappear in F2 when heterozygotes are crossed.

During non-disjunction occurring in anaphase 1 of meiosis, what fails to separate?

homologues fail to separate

Test Cross:

is a kind of backcross in which the parent has a known genotype (homozygous recessive)

Manx cats are heterozygous for a dominant mutation that results in no tails (or very short tails), large hind legs, and a distinctive gait. The mating of two Manx cats yields two Manx kittens for each normal, long-tailed kitten, rather than three-to-one as would be predicted from Mendelian genetics. Therefore, the mutation causing the Manx cat phenotype is likely a(n) __________ allele.

lethal The predicted segregation pattern in the F2 generation is 1/4 normal (homozygous), 1/2 Manx phenotype (heterozygous), an 1/4 embryonic lethal (homozygous for the Manx allele)

Meristic Traits:

must be counted. Equals quantitative traits. (bell curve)

Autosome:

non-sex chromosome

A phenotypic ratio of 3:1 in the offspring of a mating of two organisms heterozygous for a single trait is expected when:

the alleles segregate during meiosis. Mendel first proposed that alleles segregate from one another during the formation of gametes.

A phenotype ratio of 9:3:3:1 in the offspring of a mating of two organisms heterozygous for two traits is expected when:

the gene pairs assort independently during meiosis The parental organisms have the same phenotype, but their offspring have 4 different phenotypes. The 9: 3: 3: 1 ratio can only occur if the two traits assort independently during meiosis.

If Mendel's crosses between tall, spherical-seeded plants and short, dented-seeded plants had produced many more than 1/16 short, dented-seeded plants in the F2 generation, he might have concluded that:

the spherical seed and tall traits are linked. Linked traits do not segregate independently during gamete formation. Mendel was probably very fortunate to have selected unlinked traits to study. Had he observed linked or partially-linked traits, it would have been more difficult to arrive at his theory that alleles of different genes assort independently during gamete formation.

Aneuploidy

the wrong number of chromosomes

To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype

yy A cross with the homozygous recessive (yy) is a test cross. If the parent of unknown genotype is heterozygous (Yy), half of the offspring will have the recessive trait. The unknown genotype could also be determined by a cross with a known heterozygote (Yy).